One of the major obstacles for an organic chemist today is the time consuming search for efficient routes in organic synthesis. The challenges for the pharmaceutical industries and the organic chemist include identification of ways of reducing time in drug development, identification of ways of creating chemical diversity, development of new synthesis routes and maybe reintroduction of old “impossible” synthetic routes. Also, it is a constant challenge to reach classes of totally new chemical entities.
Chemical reactions are often performed at elevated temperature to enhance the speed of the reaction or supply enough energy to initiate and maintain a reaction. Microwaves assisted chemistry offers a way to perform reaction processes and circumventing at least some of the above-mentioned problems, namely                speeding up the reaction time with several orders of magnitudes,        improving the yield of chemical reactions,        offering higher purity of the resulting product due to rapid heating and thereby avoiding impurities from side reactions, and        performing reactions that are not possible with conventional thermal heating techniques.        
Recent developments have lead towards apparatuses comprising a microwave generator, a separate applicator for holding the sample to be treated, and a waveguide leading the generated microwave radiation from the generator and coupling it into the applicator. Even if the system consists of a 2450 MHz, TE10 waveguide to which a magnetron generator is connected in one end and the sample container is in the other end, there is a need for a matching device in the form of at least a metal post or iris between the generator and load, in order to achieve a reasonable efficiency.
When coupling electromagnetic radiation such as microwaves from a source to an applicator, it is important to match the waveguide impedance and the applicator impedance (with sample) in order to achieve a good transfer of power. However, the dielectric properties of the sample will influence drastically upon the impedance of the applicator, as well as its electrical size, and the dielectric properties of the sample often change considerably with both temperature and applied frequency. Thus, an impedance mismatch between the source and the applicator will often occur and the coupling and thereby the heating process becomes less efficient and difficult to predict
U.S. Pat. No. 5,837,978 discloses a microwave heating system applying a resonant multimode applicator comprising means for impedance matching during a heating process in order to achieve resonance of the system. The matching or tuning is carried out by adjusting the height of the applicator and the position of a microwave antenna/probe in the applicator (see e.g. column 7, lines 17–24 or column 8, lines 33–39).
In multimode cavities, the electric field is a superposition of several longitudinal modes and several transverse modes. When a multimode applicator is tuned to resonance, one changes the balance between these modes and thereby the spatial energy distribution. The energy distribution is therefore neither spatially uniform nor constant during the heating process, which makes it difficult to obtain reproducible results since a small change of the position or size of the sample, or a resonance tuning (performed by the user or by a change in the dielectric properties of the sample), will resulting different power absorption. Rotation of the sample in the oven does not significantly improve the reproducibility, since some of the modes, as a matter of fact most of the modes in a true multimode system, have a tendency to heat the outer parts of the sample more strongly. This leads to a position dependent heating of the sample, which is also dependent upon the resonance tuning. The samples used in microwave chemistry typically have volumes ranging from a few μL to ˜10 mL, and it is therefore crucial to have a uniform and known energy distribution.
WO 99/17588 discloses a microwave oven having a conductive member for controlling the feeding of microwave power from a waveguide to a multimode applicator. The conductive member acts as a diffracting resonator and provides a local region with a particular field pattern. When the member is rotated, the field changes, giving rise to an advantageous feeding of microwave power to the multimode applicator. The conductive member is preferably an elliptic ring member.
EP 552 807 A1 discloses a similar microwave oven having a rotatable metal reflector in a waveguide for impedance matching between the waveguide and a heating chamber.
Single mode applicator resonators offer a possibility of high field intensities, high efficiency and uniform energy distributions. The use of single mode applicators have been reported, see e.g. U.S. Pat. Nos. 5,393,492 and 4,681,740. However, since the dielectric properties of the sample changes the resonance frequency and since magnetrons usually only provide a fixed frequency or only a minor adjustment around the centre frequency of the magnetron, the generated frequency and the resonance frequency of the mode will detune as the sample heats. Thereby the high intensity in the resonant mode is lost.
U.S. Pat. No. 2,427,100 and NL Octrooi No. 75431 both discloses means for adjusting the point impedance, or wave reflection, in microwave waveguide transmission systems by having a conducting deflector rotatably mounted in the waveguide. Both systems tune the waveguide system by introducing a reactance into the waveguide. Note that only the scattering, i.e. reflection of a specific waveguide mode, is affected.
U.S. Pat. No. 4,777,336 discloses a method for controlling heating patterns in single or multimode applicators by tuning the applicator using a probe or sliding shorting plates within the applicator.
It is generally a disadvantage of the multimode applicators that the spatial energy distribution changes when it is tuned for impedance matching.
It is another disadvantage of the multimode applicators that the applicator has a non-uniform energy distribution.
It is a further disadvantage of the multimode applicators that the multimode heating pattern is not reproducible (i.e. very sensitive to its dimensions) and may change as a function of the temperature of the load.
It is a disadvantage of the prior art single mode applicator apparatuses that there are no efficient and durable means for tuning the resonance frequency in response to the dielectric properties of the load, since galvanic contacting by for example screw posts or metal vanes is needed for efficient control of also small coupling factors and the air distances to the waveguide walls tend to become so small that there is a risk of arcing.